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TS870S cap mod : Audio input impedance

Posted: Fri May 24, 2013 3:16 pm
by JJ2013
Hi y'all Audio Heads!

I haven't yet been able to establish what the audio input impedance is after the cap mod on the TS870S? I have a 4.7uF cap parallel to the original 0.1 uF ("C131") now, but has anyone measured what the audio input impedance is?

With regard to signal voltage level, I guess I should be able to determine, but have yet to do so.

I would really appreciate if someone could provide answer to the above questions, especially on the audio input impedance?

Have a great Memorial Day weekend!!


Posted: Sun May 26, 2013 7:28 am
by Voodoo Guru
Howdy JJ,
We have never actually measured the Input Impedance of that circuit as it was never a concern or needed. Generally, an Unbalanced Audio Input is from medium to high impedance.

Just looking at the schematic, our guess is a high impedance due to the capacitive reactance of the capacitors in the circuit and there is no lower value resistor to ground to set it. Not real sure on this, as we have never actually measured it.

Take Care,

Audio input impedance ACC2/TS870S

Posted: Sun May 26, 2013 11:07 am
by JJ2013
Hi VooDoo Guru!!

I got an answer from W9AC, Paul, who originally came up with this cap mod idea unless I am wrong. From his answer I infer that he may not have measured the input impedance either.

"To calculate, simply think of C131 as a short. Then look into the circuit. This is part of a process called "Thevanizing a circuit." So, that leaves R137 in parallel with R133. The input Z then is a bit less than 1K. Drive it with 600-ohms or less. At audio frequencies, ignore C128 and C129. They are affected by RF frequencies and are there to help with RFI."

I also learnt from a Chief Engineer who has 40 years of analog design under his belt and is published and holds many patents that "a 1K input impedance doesn't exactly suggest that this is a line level input based on IEC standards. According to international standards (IEC) that input impedance would meet standards for a mic-level input but would fall short of line-level, which requires 10 k-ohm or greater for professional or 22 k-ohm or greater for consumer equipment."

Best 73s!


Posted: Sun May 26, 2013 3:03 pm
by Voodoo Guru
Well there you go, Paul would know!

Don't get to caught up in all that though... it's easy to do.

As long as you have enough drive, good isolation, and a way to attenuate/adjust the levels to the input, it should work just fine for you.
We have never had a problem with Hum or S/N ratio with any of the typical pro audio pieces of equipment into the Modified ACC2 Input on the 870.

Good Luck,

Audio input impedance ACC2/TS870S

Posted: Sun May 26, 2013 3:10 pm
by JJ2013
Hi Voodoo Guru!

I appreciate the response and I tell you, it is easy to fall prey to second-guessing everything.

Perhaps the "hum" that I am hearing is somehow related to my perception of the TS870S having gotten a little more "noisy" on receive after the cap mod was done? I am not exactly a complete "klutz" when it comes to being careful so perhaps the "noise" on receive has something to do with the way I did the cap mod.....? When I feed audio in through the front mic jack there is absolutely no "hum" whatsoever so clearly it has something to with the audio chain when feeding audio in through the back! I am thinking of getting a Jensen audio isolating transformer too to put between the last piece in my audio "chain" and the ACC2.

73 de NI2M